Machines, including on- and off-highway haul and vocational trucks, wheel loaders, motor graders, and other types of heavy machinery generally include a multi-speed, bidirectional, mechanical transmission drivingly coupled to an engine by way of a torque converter assembly. The torque converter assembly multiplies and/or absorbs torque fluctuations transmitted between the engine and the traction devices of the machine by allowing slippage between an output shaft of the engine and an input shaft of the transmission. To improve fuel consumption and reduce power loss due to the slippage, the torque converter assembly typically includes a lockup clutch for mechanically coupling the engine output and transmission input shafts at high travel speeds. The torque converter assembly may also include an impeller clutch to allow the operator to have another degree of freedom in determining the slippage between the output shaft of the engine and the input shaft of the transmission. The impeller clutch allows for high engine speeds while limiting the torque that is transmitted through the transmission, thus allowing some of the torque to be directed for other purposes.
During operation of the machine, the traction devices can be decoupled from the engine to allow the machine to coast down an incline and accelerate quickly without driving the engine and incurring associated power losses. This decoupling can be accomplished by selecting a neutral condition of the transmission, decreasing the degree of engagement of the impeller clutch, or releasing the lockup clutch. However, in some situations, decoupling the traction devices from the engine when traveling too fast can cause the machine to accelerate to a speed that exceeds a braking capacity of the machine. In these situations, excessive brake wear and damage to the machine may be possible.
One method of preventing coasting to an excessive speed is disclosed in U.S. Pat. No. 4,373,619 (the '619 patent) issued to Schritt et al. on Feb. 15, 1983. The '619 patent discloses a control system for a shiftable multi-speed hydraulically operated transmission system. The transmission system includes internal clutches, which are selectively operated to effect shifting in neutral, reverse, or forward speed ranges. The transmission system also includes a manually operable multi-position speed range selector switch, sensing devices for ascertaining transmission input and output speeds, and other condition responsive switches. The transmission system also includes logic circuits therein for receiving input signal information from the switches and sensing devices, and for providing output signals to effect automatic shifting of the transmission to a selected speed range.
The transmission system of the '619 patent provides operational safeguards. Specifically, in response to an operator input selecting a desired shift to neutral, the transmission system compares a vehicle speed to a non-attainment threshold. If the vehicle speed exceeds the non-attainment threshold, the desired shift to neutral is inhibited. As the vehicle speed reduces to below the non-attainment threshold, the desired shift to neutral is accommodated. In this manner, a decoupling of traction devices from the engine that could result in overspeed of the vehicle may be prevented.
Although the transmission system of the '619 patent may minimize the likelihood of overspeed due to coasting, it may still be possible for an operator to inappropriately decouple the traction devices from the engine. Specifically, the operator may still be able to depress an impeller clutch input device causing the lockup clutch to disengage and/or decrease the degree of engagement of the impeller clutch. A decrease in the engagement of the impeller clutch results in less torque being conducted between the engine and traction devices, thereby allowing the vehicle to coast more freely. In addition, it may also still be possible for the lockup clutch to disengage, again allowing the vehicle to coast more freely. As described above, this increased ability to coast can result in the vehicle overspeeding.